1. Field of the Invention
This invention relates to the production of gypsum wallboard and a novel composition and method for improving the setting time and strength of the gypsum in the wallboard.
2. Description of the Prior Art
To manufacture gypsum wallboard an aqueous slurry of calcined gypsum (notably calcium sulphate hemihydrate) is continuously spread between upper and lower paper sheets. The continuous product thus formed is conveyed on a continuous moving belt until the slurry has set. The strip or sheet is then cut to form boards of prescribed length, after which they are then passed through an oven or kiln to be dried until the excess water in the gypsum board has evaporated. In the production of gypsum wallboard, it is customary to add various substances to the slurry for various purposes. For example, it is customary to lighten the weight of the calcined gypsum slurry by incorporating foaming agents to provide a degree of aeration which lowers the density of the final wallboard.
It is also customary to decrease the setting time of the calcined gypsum slurry by incorporating gypsum set accelerators. Calcium sulphate hemihydrate mixed with an appropriate amount of water will normally set in about 25 minutes to a hardness that enables wallboards made from such gypsum to be handled. In modern wallboard manufacturing plants, where a high production rate is necessary, 25 minutes is too long, and it has been customary to reduce the setting time of the aqueous slurry to about 5 to 8 minutes by incorporating a gypsum set accelerator. Freshly ground gypsum is a well-known gypsum set accelerator but has a relatively short shelf life, and various ways and means have been sought to improve the shelf life of the ground gypsum.
For example, U.S. Pat. No. 4,019,920 discloses a milled gypsum-starch composition, U.S. Pat. No. 4,059,456 describes a composition made up of finely ground calcium sulphate dihydrate and lignosulphonate, a waste product derived from the sulphite pulping of wood and Canadian Pat. No. 999,310 proposes a finely particulate intimate admixture of gypsum and a long chain fatty carboxylic acid containing at least 12 carbon atoms, or a salt thereof. Other publications propose the use of sugar or soluble dextrin as stabilising agents for ground gypsum to be used as a gypsum set accelerator, but these materials are expensive.